The present invention generally relates to a polarographic cell and more particularly, to an enzyme electrode which is capable of quickly and readily measuring the substrate concentration of enzyme, and moreover, can be used repeatedly.
More specifically, the present invention relates to an enzyme electrode, which can remove influence of various interfering materials contained in a liquid to be tested or test solution (referred to as a test solution hereinbelow) which materials interfere with the electrochemical detection during measurement of the substrate concentration in the test solution by the use of a polarographic cell.
In one example for measuring the concentration of the substrate, which is a material subjected to the peculiar catalytic reaction of the enzyme, with the use of a polarographic cell, glucose is oxidized, through reaction of an oxide reductase enzyme such as glucose oxidase, to produce hydrogen peroxide H.sub.2 O.sub.2 as shown in the following equations (1) and (2). Then, H.sub.2 O.sub.2 thus produced is oxidized by the use of a platinum electrode or the like, and the concentration of the substrate (glucose) can be found from the oxidation current value obtained at this time. ##EQU1## EQU H.sub.2 O.sub.2 .increment.2H.sup.+ +O.sub.2 +2e (2)
In order to form an enzyme electrode for measuring the concentration of a repeatedly usable substrate through application of this principle, in the above instance, for example, it is required to immobilize the water-soluble glucose oxidase on or in the vicinity of a current collector such as a platinum electrode or the like. For the immobilization of an enzyme, there have generally been employed various methods, for example, a method of using a high polymeric organic membrane such as cellulose or the like as an immobilizing carrier.
On the other hand, there is the problem of an interfering material contained in the test solution for the measurement of the substrate concentration by such an enzyme electrode. For example, during measurement of glucose in the blood, various coexisting materials such as uric acid, ascorbic acid, etc. contained therein are electrochemically oxidized directly on the electrode. Namely, since the coexisting materials are simultaneously oxidized in the oxidation of H.sub.2 O.sub.2 on the electrode as shown in the equation (2), errors are undesirably involved in the current value to be obtained.
In order to overcome the disadvantage as described above, there has conventionally been proposed, for example, in U.S. Pat. No. 3,539,455, an enzyme electrode in which countermeasures are taken against the interfering materials referred to above. In this prior art, with employment of two platinum anodes, enzyme is immobilized only to one of the anodes, and by subtracting current values for the both, the influence of the interfering material is to be compensated. However, the above known method has a disadvantage in that it is very difficult to properly balance response characteristics of the two electrodes (platinum anodes).
In other prior arts proposed, for example, in U.S. Pat. Nos. 3,979,274 and 4,240,889, it is intended to prevent uric acid, ascorbic acid and the like from being diffused into the platinum electrode by disposing membranes of cellulose acetate, silicone rubber or the like at the side of the platinum anode contacting the test solution. The effect of the method described in the above prior arts with respect to the interfering materials depends upon selectivity of the membrane for H.sub.2 O.sub.2 and the interfering material. Namely, the extremely fine membrane made of cellulose acetate or the like is set before the platinum electrode (on test solution side) for detecting H.sub.2 O.sub.2 so as to select the interfering material such as ascorbic acid or the like from H.sub.2 O.sub.2 by the use of this membrane. However, with the fine membranes in the prior arts as described above, diffusion of H.sub.2 O.sub.2 is naturally restricted which reduces the sensitivity and response speed. Meanwhile, since the selectivity of these membranes is relative with respect to H.sub.2 O.sub.2 and the interfering material, it is difficult to completely prevent such undesirable materials from interferring. It is considered that the effect becomes larger, with an increase of the membrane thickness to a certain extent, but a further reduction in the response-current (reduction in sensitivity) and response speed may result undesirably.